An exhibition at London University's School of Oriental and African Studies (SOAS) is an ambitious but flawed attempt to understand Zoroastrianism, which claims to be the world's first monotheistic faith and the state religion of the first global superpower, ancient Persia. Both claims are shrouded in antiquity and hard to verify, but they are deeply embedded in the psyche of one of the world's tiniest but most successful minorities, the Parsees.

Sometimes called "the Jews of the East", the Parsees too have a history of a very creative diaspora. With their origins in Persia (or Pars, hence the name), they migrated to Gujarat, on the west coast of India, during the eighth to tenth centuries AD. One of their founding myths in India was that they were refugees from Islamic religious persecution in Iran after its Zoroastrian kingdom was conquered by Arabs. But the historical evidence shows that the migrations took place over a period of centuries, motivated as much by trade as by religious dissent.

The SOAS exhibition, with its lack of any narrative structure, sheds little light on such questions. Nor does it define the role of Zoroastrianism in Persia's mighty Achaemenid Empire, which ruled most of the Middle Eastern world at its peak in the fifth to sixth centuries BC. The first imperial inscriptions honour Ahura Mazda, the Zoroastrian Lord of Light, not as a monotheistic Jehovah-like figure, but as the chief deity in a land which also worshipped pagan gods and goddesses, like their Greek contemporaries. One has no sense in this exhibition, with its small, dingy spaces and unreadable labels, of the majestic friezes and colonnades of Persepolis and Susa.

The existence of the prophet Zoroaster (aka Zarathustra), unlike that of Christ and Muhammad, is undocumented, resting solely on oral traditions that date him to prehistoric times. But that doesn't prevent his Parsee followers from depicting him visually in their fire-temples and living-rooms as a benign, Christ-like figure with a flowing beard and biblical robes. Such portraits apart, the Parsees are iconoclasts: only the sacred flame, burning at the heart of their temples, symbolises the presence of Ahura Mazda. There are no congregational prayers and remarkably few rules and observances. The main thrust of the religion is abstract and ethical: good thoughts, good words, good deeds.

Growing up in India in the 1950s as the child of a rare Parsee-Hindu love match, I was acutely conscious that the Parsee sense of ethical superiority was closely linked with a largely mythical belief in the community's racial purity. Despite obvious visual evidence of Parsee intermarriage with indigenous Indians, there was a tacit faith in the superiority of the ideal Parsee type: fair-skinned, even blue-eyed, with aquiline noses. My grandmother closely fitted that ideal, and I still remember her scrubbing me in the bath in a vain attempt to lighten my brown, Hindu colouring.